Thursday, February 21, 2013

Grinding Eggshells

"We should do something with all those eggshells," we'd been saying to each other in reference to the four dozen light brown hemispheres 'drying' around the kitchen sink.  'Drying' is a bit of a euphemism because most of them had already been dry for over a week.  But...the problem of what to do with all the eggshells is a tricky one for us.  We don't have a full-size compost pile to toss them on.  They take forever to break down in the worm bin, and if we don't crush them up beforehand, each half-shell spontaneously turns into a snake pit, except the pit is very small and the snakes are actually worms.

But it turns out that eggshells, which are almost entirely calcium carbonate (CaCO3), have a plethora of uses around the homestead, provided we can grind them up small enough.  For example, in addition to being fine for the worm bin when ground up, eggshells can:

The trick is how to get the shells ground fine enough, since some of these applications need very small particles.  Crushing them with our bare hands doesn't do it, and the mortar and pestle even makes a pretty hard time of it. Our electric food processor doesn't do too much better than the mortar and pestle, although many other people seem to have better luck with similar electronic gadgets like coffee grinders and blenders.  But, we also like non-electric methods if we can help it!  We're still working on our technique, but we came up with a process the other day that can handle our modest egg shell supply without too much trouble.

Also, it's probably a good idea to sterilize the shells before ingesting them as supplements or toothpaste.  That can be done in an oven above 250 °F or so, using leftover heat from baking something else, before or after the grinding.  See also the comment below the pictutorial.

Here they are to start.  27 eggs in total, or 54 half shells.

Put one or two shells in a mortar and grind it up with the pestle to about 1/16" pieces.  With only one or two shells, it should take 5-10 seconds. Larger loads take a lot longer.
Here's the 1/16" pieces.  If you leave them bigger, the next step doesn't work as well because the pieces make bridges and don't fall into the mill blades nicely.
Add the eggshells to a grain mill and start grinding on a fairly coarse setting.
The output should look kind of like flour.  It can help to keep the hopper fairly full, and tap it frequently to shake down the pieces.
Here it is, all done.  The 27 eggshells took about an hour to process from start to finish (not including the drying).  A little longer than we would like, but it makes a nice even product.
We got a little under a cup of ground egg shell from the 27 eggs, and we also got a good arm workout in.  The powder can be used straight away in any of the applications mentioned above, but make sure to add the right amounts!

Note that if you want to get relatively pure calcium carbonate from the eggshells, without the protein and other stuff from the membranes, you should be able to use your self-cleaning oven.  The 'clean' setting heats up to around 900 °F (500 °C), which should be enough to burn off all the carbonaceous stuff in your oven (i.e., food gunk), leaving the minerals (ash) behind.  But since eggshells are mostly mineral to begin with, the 'clean' cycle will just burn off the minor components.  We've used this same technique to clean smoke residue off pots and pans that we took camping--but don't use it on anything with wood, plastic, or nonstick components!  There's also a risk that you could burn off the seasoning from your cast iron or stoneware items, so it's best to use stainless steel vessels if you can.

What do you use egg shells for around your house?  Have you found a good way to grind them up with minimal effort? Tell us about it in the comments section below!

EDIT: The clean cycle on some ovens might not get hot enough to turn crushed eggshells into a nice white powder, so you'll have to experiment with yours if you really want the calcium carbonate.  Also, we've found it preferable to use a sheet of aluminum foil instead of a steel pot, since the clean cycle can be a little harsh on some pots and pans.


  1. coffee grinder full speed and shake rapid

  2. Better use a stone made mortar only since there is a proof that contact with a metal diminishes ionization of calcium in eggshells